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Double blossom (Cercosporella rubi), also known as rosette, is a common fungal disease of blackberry. It may occur on raspberry, but disease is seldom severe. Emerging shoots appear smaller than normal and stunted, and leaves turn pale to bronze. Witches’ broom symptoms develop, and more than one broom can occur on a single cane. Flower buds expand to larger-than-normal and are misshapen; sepals turn into leaves. Fungal spore masses can be visible on flowers; fruit does not develop. The fungus overwinters in infected primocane buds, and symptoms develop the next spring.

Double blossom affecting leaf and flower development on floricanes.

(Photo: Barbara Smith, USDA-ARS)

Double blossom affecting blossoms.

(Photo: Barbara Smith, USDA-ARS)

Witch's broom symptom.

(Photo: Michele Stanton, University of Kentucky)

Sterile blossoms.

(Photo: Michele Stanton, University of Kentucky)

Management:

• Consider planting resistant cultivars.
• Rogue (completely remove) infected plants.
• Practice bi-annual cropping.
• Fungicides are not effective.

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